1. Field of the Invention
This invention relates to filters for alternative current circuits, and particularly to passive filters to remove unwanted harmonics generated on an alternating current power line.
2. Description of Related Art
Harmonic filters will increasingly become an essential part of a power system in order to comply with strict codes on harmonics adopted by electric utilities based on the recent Institute of Electrical and Electronics Engineers standard 519. This is particularly true when a Variable Frequency Drive (VFD), which can be a substantial generator of harmonics, is being attached to the power system. All VFDs generate current harmonics which can create problems with an industry's power supply system and also with the utility's power grid. Harmonics also have detrimental effect on telephones and other means of communications. The harmonics generated by VFDs can be grouped into two categories:
(i) Harmonic current generated by the pulsed current pattern at the input of a diode bridge rectifier; and PA1 (ii) Flat voltage top created by the phenomenon of dc bus capacitor being charged by the ac to dc rectifier block in the absence of any series impedance, which causes objectionable voltage harmonics.
In order to study the harmonic effect on a power system, it is essential to define a term by which to measure this harmonic. The most widely accepted term for measuring the harmonic content in a given waveform is the Total Harmonic Distortion (THD). THD is used to define the effect of harmonics on the power system. It is defined as the ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental: ##EQU1##
If the quantity under consideration is voltage, then the definition refers to the Total Voltage Harmonic Distortion or TVHD. If the quantity under consideration is current, the definition then refers to the Total Current Harmonic Distortion or TCHD.
The current harmonics generated by the diode rectifier bridge configuration with a dc bus capacitor (as illustrated in FIG. 1) can be reduced by designing harmonic filters which trap the harmonics generated and thereby do not allow it to travel to the ac power source.
A Variable Frequency Drive (VFD) which uses a thyristor rectifier bridge to regulate the dc bus voltage (as illustrated in FIG. 2) will create notches in the voltage waveform. This is because of the unavoidable system inductance to which a drive is connected. Notching of the voltage happens when an incoming thyristor takes over conduction from an outgoing thyristor. There is a period when both the incoming and the outgoing thyristors conduct creating a temporary line-to-line short-circuit which appears as a notch in the voltage waveform. The notching can cause resonance in an already existing resonance condition which hitherto was unnoticed as there was no excitation for it. The effect of notching is normally reduced on large drives by having either line reactors or input transformers. The reactance of these reactors act as a voltage divider circuit with the supply impedance ratioed to the line reactance such that the notch depth is controlled at 10% or less of the line-to-line voltage. As the reactance of the reactor increases, the notch depth decreases but the overlap duration during commutation increases.
The notching effect is absent in the diode rectifier bridges illustrated in FIG. 1, since there is no firing angle control to regulate the dc bus voltage. Also, most modern rectifier diodes have low turn-off times (typically 7 us) compared to thyristors (typically 100 us) of similar rating so that the overlap duration is minimal and has no notching effect on the input voltage.
In order to reduce the current harmonics, tuned filters between the power supply line and ground are often used. A three-phase, 6-pulse rectifier on the input of the VFD generates a large fifth current harmonic. It is customary to design a harmonic filter tuned to a frequency close to the fifth harmonic. However, a large fifth harmonic filter will offer only capacitive impedance at the fundamental frequency (50 or 60 Hz) and cause an overvoltage across the line-to-line terminals. This overvoltage can be typically as high as 15%. This is a serious drawback as any other load connected on the same network will experience a rise in voltage thereby causing frequent trippings and other fall-outs.
A further difficulty arises in protecting the filter components. Most protective relays are sensitive to the fundamental 60 Hz (or 50 Hz) component while they are insensitive to the 5th, 7th, or the 11th harmonic components. Thus the filter can be easily overloaded by harmonic currents being imported from other pieces of equipment or from the utility due to their designed low harmonic impedance.
In addition to the 5th harmonic component, a 6-pulse rectifier unit with a dc bus capacitor generates strong 7th, and 11th harmonic currents. In order to bring down the overall Total Current Harmonic Distortion (TCHD) to an acceptable level, one has to provide tuned filters for filtering those harmonic components as well. It is a general practice to provide 5th and 7th tuned harmonic filters. To filter out the higher harmonic components, a high-pass filter is provided. A typical filter structure used commonly in the industry is shown in FIG. 3.
The basic design strategy adopted for designing each of the filter components shown in FIG. 3 is well known and has been thoroughly ,investigated and documented in the prior art. It is clear from the discussions in the prior art that there is a serious danger of using the filter combination of FIG. 3 if one fails to consider the network impedance to which the filter is to be connected. The combination of the filter with the power system network can set up a new resonance frequency which can be excited in the presence of the wade harmonic spectrum generally associated with variable frequency drives. Thus a conventional tuned filter may cause more problems that it solves. Independent authorities have arrived at the same conclusion regarding the care needed in employing conventional tuned filters for harmonic reduction as evidenced in many articles published in leading journals.
An alternative to using a number of tuned passive filters is to use an active low-pass filter in the power supply line. Such a low-pass scheme is described in "An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers" by Prasad and Ziogas (IEEE Transactions on Power Engineering, Volume 6, Number 1, January 1991). However, boost inductors and an active control circuit are necessary for the satisfactory operation of the filter, increasing its complexity.